This invention relates to a cooling scheme for use in an airfoil, such as a turbine blade, or vane, wherein a serpentine cooling channel is supplemented by an additional cooling microcircuit at an area of high stress.
Turbine blades are utilized in gas turbine engines. As known, a turbine blade typically includes a platform, with an airfoil shape extending above the platform. The airfoil is curved, extending from a leading edge to a trailing edge, and between a pressure wall and a suction wall.
Cooling circuits are formed within the airfoil body to circulate cooling fluid, typically air. One type of cooling circuit is a serpentine channel. In a serpentine channel, air flows serially through a plurality of paths, and in opposed directions. Thus, air may initially flow in a first path from a platform of a turbine blade outwardly through the airfoil and reach a position adjacent an end of the airfoil. The flow is then returned in a second path, back in an opposed direction toward the platform. Typically, the flow is again reversed back away from the platform in a third path.
The location and shape of the paths in a serpentine channel has been the subject of much design consideration.
During operation of the gas turbine engine, the cooling air flowing inside the paths is subjected to a rotational force. The interaction of the flow through the paths and this rotation results in what is known as a Coriolis force which creates internal flow circulation in the paths. Basically, the Coriolis force is proportional to the vector cross product of the velocity vector of the coolant flowing through the passage and the angular velocity vector of the rotating blade. Thus, the Coriolis effect is opposite in adjacent ones of the serpentine channel paths, dependent on whether the air flows away from, or towards, the platform.
To best utilize the currents created by the Coriolis effect, designers of airfoils have determined that the flow channels, and in particular the paths that are part of the serpentine flow path, should have a trapezoidal shape. Essentially, the Coriolis effect results in there being a primary flow direction within each of the flow channels, and then a return flow on each side of this primary flow. Since the cooling air is flowing in a particular direction, designers in the airfoil art have recognized the heat transfer of a side that will be impacted by this primary direction will be greater than on the opposed side. Thus, the trapezoidal shapes have been designed to ensure that a larger side of the cooling channel will be impacted by the primary flow direction. As mentioned, this primary flow direction will be different in the first and third paths described above, than it is in the second path. With such trapezoidal-shaped paths for a serpentine flow, the wall adjacent the smaller side of the trapezoid has less cooling surface area and less cooling efficiency.
In addition, in some cases, the smaller side of the trapezoidal path may be spaced from a wall by a relatively great distance. Further, the serial flow paths are typically at an area of high curvature that is subject to relatively high mechanical and thermal stresses.
For all of these reasons, the area between a smaller side of one of the trapezoidal flow paths and its facing wall is an area that could benefit from additional cooling.
Recently, the assignee of the present invention has developed cooling circuits that are embedded into the wall of an airfoil, which have been called microcircuits. These microcircuits are disclosed in a co-pending U.S. patent application Ser. No. 10/637,352, entitled “Microcircuit Airfoil Main Body,” and which was filed on Aug. 8, 2003. These microcircuits have never been disclosed for use at the above area between the smaller side of the trapezoidal flow path in a serpentine channel and the facing wall.